When terms changed during negotiation, restate the final agreed figures precisely and reference the date of the revised offer. This is the document that prevents the old number reappearing in your contract.
Most employers ask for a signed offer letter, but a written acceptance email alongside it creates a clear timestamped record of the terms as you understood them.
Yes. It costs one sentence and catches discrepancies between what was discussed and what was documented, while the offer is still easy to correct.
Practically, no. Negotiate before you accept — once you have accepted in writing your leverage is largely gone.